1. Field of the Invention
The present invention relates to a magneto-optically modulating system, and more particularly to a method of and apparatus for magneto-optically modulating light by utilizing a magneto-optic effect element which is capable of rotating the plane of polarization of the linearly-polarized light while the polarized light passes through the magneto-optic effect element.
2. Description of the Related Art
There have so far been proposed a wide variety of magneto-optically modulating systems which can be adapted, for example, to position sensing apparatuses such as linear scales or linear/rotary encoders. A typical example of the position sensing apparatus is adapted to monitor an object, e.g., a rotary toothed gear, and generally comprises a detecting unit constituted by, for example, a Hall effect element, magneto-resistance effect element, an inductive coil type element or the like, to detect the magnetic field produced by the object which has been magnetized or is made of a magnetic substance. The position sensing apparatus thus constructed can measure the object by a relative position between the object and the apparatus, a rotational frequency of the object, the shape of the object and so forth.
One representative example of the position sensing apparatuses is shown in FIG. 21 as comprising a detecting unit constituted by the Hall effect element. In this example of the position sensing apparatus, the Hall element of the detecting unit is operated to output a signal indicative of an electric current small enough to interfere with the magnetic field. Therefore, this type of the position sensing apparatus is required to electrically amplify the signal to be produced at the highest sufficient signal-to-noise ratio (S/N). For this reason, the position sensing apparatus needs to have a large circuit to meet the above requirement.
Other than the above prior-art position sensing apparatus is proposed and shown in FIG. 22 a position sensing apparatus which comprises a detecting unit constituted by the inductive coil type element. In this example of the position sensing apparatus, the inductive coil type element can output a relatively large signal in comparison with that of the above Hall element type position sensing apparatus. This type of the position sensing apparatus, however, has a drawback in which the signal outputted from the detecting unit fails to maintain its stability because of the fact that the amplitude of the output signal is varied in accordance with the degree of modulation of the magnetic flux indicative of the magnetic field. This means the fact that, when the degree of modulation of the magnetic flux is small with the motion velocity of the toothed gear being small, the frequency of the amplitude of the outputted signal is reduced thereby causing the S/N to be extremely reduced. When, on the other hand, the motion velocity of the toothed gear becomes large, the degree of the modulation of the magnetic flux becomes large, but the output signal is induced to have an extremely high frequency to make the impedance of the inductive coil high. Therefore, it is difficult to output the signal.
Further, there has so far been proposed another position sensing apparatus of this kind, for instance, disclosed in U.S. Pat. No. 5,434,934 and developed in order to improve the above conventional position sensing apparatus. This position sensing apparatus is also adapted to modulate a polarized light and comprises a magneto-optic effect element having an internal magnetization directed to a spontaneous magnetization direction and positioned on a transmission path of the polarized light with the spontaneous magnetization direction in substantially parallel with the transmission path of the polarized light. The position sensing apparatus is operated to have the internal magnetization of the magneto-optic effect element oriented to a direction substantially perpendicular to the transmission path of the polarized light by applying a magnetic field to the magneto-optical effect element. The application of the magnetic field to the magneto-optic effect element is effected by an object. The applied magnetic field is varied in accordance with a relative positional relationship between the object and the magneto-optic effect element to have the polarized light modulated. This means that the variation of the magnetic field can be converted into an optical signal.
The prior-art position sensing apparatus, however, has encountered such a drawback that the position sensing apparatus is not available because the object to be analyzed cannot allow a magnet to be attached thereto if the object has no ferromagnetic characteristic. In order to analyze the object with this kind of the prior-art position sensing apparatus, it is necessary that a magnet be attached to the object. For instance, the position sensing apparatus is assumed to detect a rotational frequency of a toothed wheel having a rotary shaft when the toothed wheel is rotated about the rotary shaft. In this case, it is necessary for the toothed wheel to have a predetermined pattern of the magnetic field produced thereon in order to analyze a rotational frequency of the toothed wheel on the basis of the pattern of the magnetic field of the toothed wheel. When the toothed wheel is assembled with a mechanical apparatus and repeatedly operated for a period of time long enough to produce iron dust or powder worn from the mechanical parts forming part of the apparatus, the attached magnet is liable to attract the mechanically worn iron dust or powder thereby causing a trouble on the mechanical apparatus. Moreover, the toothed wheel tends to lose a balance of weight under the influence of the magnetic field of the magnet when the toothed wheel is rotated about the rotary shaft at a high speed.